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Northwest Geology Field Trips, by Dave Tucker, is licensed under a Creative Commons Attribution-Noncommercial- Share Alike 3.0 United States License. You can use what you find here, repost it with attribution to the author, "remix" it for your own purposes, but may not use it with the intent of making money off of it.

EDUCATORS: Please feel free to use anything you find here that is useful to your mission educating people about Earth science. E-mail me if it would help to have a larger or higher-resolution version of any of the images. tuckerd at geol dot wwu dot edu

By Dave Tucker; science contributions by Doug McKeever and Wes Gannaway Original heads up and photos by Darryl Howe

detail of bivalve fossil in the Leschi Erratic. Darryl Howe photo.

An interesting fossiliferous erratic has been found following recent trail work in the upper part of Leschi Park in Seattle. Location is along a new section of trail just west of Lake Washington Blvd and south of Yesler Way. It is clearly visible directly west and uphill in the woods (recently cleared of most undergrowth by trail crews) from the small parking lot by the tennis courts. The rock is not huge, but one of the more intriguing ones I’ve heard of since sending out the ‘cool erratic call out’ to website visitors. The erratic is chock full of bivalve, perhaps brachiopod, fossils. It looks similar to Nooksack Group rocks that Doug and Dave have seen in the Mt. Baker area. The fossil content also argues for a source in the Harrison Lake area of southern BC. We expound further about these potential source areas below. The rock is fine-grained (very fine sand or silt) and appears to be well sorted and not particularly well stratified. The rock is weathered, as one would expect from sitting in the woods in a humid climate for about 13 or 14,000 years since being dumped by ice. Curious folks have hammered at a few chunks that are lying nearby. This is a fascinating find, the likes of which none of the authors have seen among several thousand erratics personally observed from the Fraser Glaciation. It is worth further investigation by paleontologists and sedimentary petrologists.

The rock is full of fossils. Darryl Howe photo.

The Nooksack Gp. rocks are known from the vicinity of Mount Baker; not all are fossiliferous. The best known fossil localities are from the argillite at Fossil Creek (south of Church Mountain) and in bedded siltstones on Chowder Ridge, at the south end of Skyline and Cougar Divides. If this erratic is from the Nooksack Group, it would most likely have been carried down the Skagit (or conceivably but less likely via the Nooksack) and into the main stem Vashon ice covering the future Puget Lowland. That is at least 95 miles in a reasonable path via the Skagit. It may have floated here in an iceberg as the ice deteriorated and receded northward.

The erratic lies beside a newly built trail. Darryl Howe photo.

The Nooksack Gp rocks are the polygons labeled 'N' in this clip from the Mount Baker 30 x 60 USGS map. Click to enlarge.

Wes Gannaway, a paleontologist in Bellingham, suggests the source area for the bivalve-bearing Leschi erratic could be the rocks around Harrison Lake, BC. He has only seen the photos posted here of the fossils in the erratic, so this is remains a hypothesis only, but it is a reasonable one.

Wes wrote: “The gray siltstones are along the shore of Harrison Lake and contain
an assemblage of ammonites and clams including the Buchia crassicola, which
I believe are the dominant Buchia species at Fossil Creek in the Nooksack
Group. The formation borders on the Cretaceous/Jurassic boundary, age
equivalent to the Nooksack Group. The fossils are abundant in the Mysterious
Creek Formation. C.H. Crickmay wrote a paper in 1930 on the locality. His
paper names them Aucella sp. and Jeletzky named them Buchia, although
I don’t know enough about the species to identify which one is in the
Leschi Park boulder. Aucella is also found in the Jackass Mountain area. Someone with
the knowledge of the species should take a look at the fossils in the
boulder. I am just looking at the photo and using the general shape and rock
color to call it Buchia from Harrison Lake.”

Thanks for that, Wes. Here’s more information: (ain’t the web wonderful?)Here’s another blogger’s post about fossil hunting in the Mysterious Creek Formation. And this blog has photos of large Buchia fossils, including this one.

Further web investigation turns up this abstract of a 1983 UBC MSc thesis by Andrew Arthur on stratigraphy and fossils in the Harrison Lake area. If you follow the above link to the thesis abstract, you’ll find a download file of the entire thesis as a pdf. In it are many photos and drawings of fossils, mostly ammonites, found in the Mysterious Creek and related formations.

As a source region for the Leschi erratic, the Harrison Lake area makes good sense. The Vashon ice flowed south right through there. George Mustoe told me that he picked up a Buchia-laden erratic on the beach at Larrabee Park back in the 1960s. That surely would not have come from the Nooksack Group, although the correlative rocks at Harrison Lake would be a reasonable candidate.

Any paleontologists out there who want to take a crack at this problem?

If anyone has more photos or insights into the origin of this unusual erratic, please write to:

email address to send reports and photos. I'll credit you.

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9 Responses

Dave, the posted photo by Maria Van Nus of the Buchia fossils causes me to think “yes, that looks very similar to what is visible in the Leschi Park erratic.” Again, I am a far cry from a paleontologist or a sedimentary petrologist, but it makes sense that the source could very likely be the Mysterious Creek Formation. My reasoning is that given abundant geomorphic evidence that Vashon Stade glaciation affecting the Puget Lowland came from the north more than from the northeast. An erratic this far south certainly would most likely be from the earlier Vashon Stade rather than from the later Sumas Stade, for which evidence suggests ice flow more from the northeast, down the Fraser Valley. However, the plot thickens……where is Harrison Lake?…….on the north side of the Fraser Valley! Could this erratic have been ice-rafted as you suggest as a possibility? If so, could it have been carried well beyond the terminus of the Sumas Stade ice? What is the specific glacial formation and age of the material the Leschi erratic rests on?
Isn’t it interesting how a random observation can open up so many other questions?

Thanks for taking an interest. Our erractic in Leschi definately looks like it has brachiopod fossils for the most part. I done believe the image from Harrison Lake matches the type of bivalve or color of siltstone. I vote for the Nooksack Group….. but HEY….I’m no expert. Anyone have more info regarding the bed material it rests on? Looks like a thick layer of silty loam soil.

The bivalve fossil in the first photo is definitely Buchia sp. (possibly B. keyserlingi), i.e. early Cretaceous, ca. 130,000,000 years old.
e-mail me if you want to learn about the 62 Buchia-bearing erratics that have been found to date in the Cortes Island area (north end of Strait of Georgia).
If you tell me how to attach images, I’ll send you a couple jpegs.

Christian,
There is some mention of similar erratics in the San Juans but I will be up near Cortez in June and would enjoy seeing some of the erratics mentioned. Please send a few images and a treasure map. Thanks, Darrell Howe dhowe@artcycled.com

I live on Whidbey Island here in Puget Sound and there is one beach where I have discovered a number of Buchia laden erratics. Just two days ago I found a my most recent specimen. They rage from 100 lbs to 5 lbs and yielded dozens of Buchia mollusks. I was unaware of the Leschi Park erratic until I began researching online in hopes of identifying what I had found. It seems the Frasier Glaciation cut right through the Harrison Lake region as well as Puget Sound and I would love to somehow confirm its source by comparing with other specimens found.

One fossil had not been previously mentioned in any articles and I was hoping to get some more answers. A beautifully preserved Baculite was nestled among the Buchia in one of the larger rocks. I’ve seen that Ammonites have been found, but Baculites have never been specifically mentioned as far as I know. There are also some cylindrical shaped fossils which I thought were possibly Belemnites, although they are not very well preserved. I will send photos to anyone interested. Thanks for everyone’s time and I look forward to hearing back.

Sam,
I’m no expert but also would enjoy learning more about the likely source and history of the erractic we have here in Leschi. There was some mention of this type located near Cortez Island as well. If you have a couple images for comparison you can send, try my address: dhowe@artcycled.com If you ever make it to the big city please reach out and see our erractic boulder.
Darrell

Sorry Eric, I don’t know anything about the Leschi formation. The rock in Ravenna Park is described as a loose block rather than an outcrop. If you find out details about the Leschi Formation that could conform to this rock, please pass that along.